1. Field of the Invention
The present invention relates to an oxide magnetic material to be used in various electronic parts, and a chip part formed with the oxide magnetic material and including a bulk-type coil part and an internal conductor for inductance to be used in a high frequency area, as well as a method of producing the oxide magnetic material, and to a method of producing the chip part.
2. Description of the Related Art
Ni—Cu—Zn based ferrite is generally used as an oxide magnetic material such as coil parts and the like to be used in a high frequency area. A powder metallurgical method is general for producing the same. According to this method, oxides such as Fe2O3, NiO, CuO, ZnO and the like to be starting materials are weighed so as to be predetermined ratios, and are then dry- or wet-mixed and crushed, and this mixed powder is temporarily baked. Subsequently the temporarily baked matter is roughly crushed and is further pulverized. In case the oxide is wet-crushed, the powder is required to be dried. Here, properties of ferrite considerably depend on the composition; therefore, the divergence of composition should be kept extremely small from the viewpoint of production control. Further, material for laminated coil is required to be baked at a lower temperature than the melting point of Ag, and the composition control is required to be at a level of 0.1 mol % of Fe2O3, NiO, Cuo or ZnO. Especially as to Fe2O3, reactivity increases as coming near to the stoichiometrical composition of ferrite, but when exceeding it, the reactivity abruptly decreases; therefore, among the main constituents of ferrite, a most serious composition control is needed.
For conventional Ni—Cu—Zn ferrite, stainless steel balls, alumina balls, zirconia balls and other have been used as medium beads in the production process, and crushed with mixing, temporarily baked, roughly crushed and further pulverized. The bulk-type coil material is normally temporarily baked and crashed such that a specific surface area is around 1.0 to 6.0 m2/g. The laminated coil material is, however, crushed for a long time because it is required to be baked at a lower temperature than the melting point of Ag, thereby to increase the specific surface area up to around 3.0 to 15.0 m2/g to heighten the reactivity of powder at low temperature.
Here, the stainless steel balls have Fe as a main constituent, and due to mechanochemical reaction during crushing, Fe2O3 which is a main constituent of Ni—Cu—Zn ferrite is increased. The increase of Fe2O3 changes the composition of Ni—Cu—Zn ferrite and causes a stabilized composition control to be difficult. That is, it is difficult to control the composition with weighed quality values. Other medium beads have a defect in abrasion resistibility, and a defect that abraded powders of the medium beads are mixed into the crushed powder as impurities.
Further, the general medium beads have the inner side of abrasion resistibility which is lower than that of the outer side. Therefore, it causes a divergence in the composition due to the difference in mixing amount of powder produced by abrasion as the production goes on, and it is impossible to obtain stable composition. The crushing for a long time will invite the increase of the amount of the abraded powder, deteriorating the property of baked material. Namely, the abraded powder mixed as impurities will deteriorate the sintering property of Ni—Cu—Zn ferrite, thereby to cause the baking temperature to be high for obtaining the density and permeability of sintered material with the neighborhood of a theoretical density. This will invite a high production cost and decrease of stability in products, and further will make it difficult to bake the sintered material at lower temperature than the melting point of Ag.
The Japanese patent No. 2708160 discloses, for the purpose of reducing the mix of abraded powder at the time of crushing, the use of balls of fully stabilized zirconia (FSZ) of high abrasion resistibility or of partially stabilized zirconia (PSZ) as medium beads for crushing Mn—Zn based ferrite. According to the method of this patent document, the zirconia balls of a diameter 0.5 to 3.0 mm are used as medium beads in the pulverizing process to extremely prevent impurities from being mixed, thereby to suppress the mixing amount to be less than 0.02 wt % relative to the main constituent. It is described in this patent document that with this method, the powder may be sintered at temperature lower by about 100 to 200° C. (that is, at approximately 1000° C.) in contrast to the temporarily baking temperature of 1200° C. or higher of the conventional art to obtain the sintered material with the neighborhood of theoretical density, thus to industrially lower the sintering temperature and reduce the production cost. Further, JP-A-7-133150 discloses a sample having ZrO of 0.01 to 3.0 wt % relative to the main constituent of Ni—Cu—Zn ferrite and baked at the temperature of 1100° C. for 1.5 hours for the purpose of providing a magnetic material of high mechanical strength.
However, the baking temperature of approximately 1000° C. described in the Japanese patent No. 2708160 will not actually reduce the baking cost and will not be adapted to the simultaneous baking with Ag of melting point of approximately 960° C. The baking temperature of approximately 1100° C. as described in JP-A-7-133150 will be further impossible for the simultaneous baking with Ag.
Further, according to the production method of the Japanese patent 2708160, medium beads of small diameter are used to reduce the amount of impurities mixed into the material due to the abrasion of the medium beads, and the temporarily baked material is crushed, taking a long time, for example, 196 hours. Therefore, the ball efficiency ([material processing amount]/[ball weight]), that is, the crashing efficiency is bad.